Generally, a semiconductor laser employs a double heterojunction structure in which an active layer is sandwiched by cladding layers having larger energy band gaps than the active layer. When a forward bias voltage is applied to the double heterojunction structure, carriers are injected into the active layer from the cladding layers and effectively confined in the active layer due to a potential barrier caused by the difference in the energy band gaps of the active layers and the cladding layers resulting in carrier recombinations that produce laser light. However, if the potential barrier is small, carriers in the active layer, especially electrons having a small effective mass, overflow into the cladding layers, whereby threshold current unfavorably increases, resulting in deterioration in laser characteristics. In order to suppress that overflow of carriers, a multiquantum barrier (hereinafter referred to as MQB) is proposed by K. Iga, H. Uenohara, and F. Koyama in Electronics Letters, Vol. 22 (1986), p. 1008.
The MQB has a multiquantum well structure comprising wells and barriers tens of Angstroms thick and utilizes a phenomenon in which an electron wave is reflected at the interface between the quantum wells and quantum barriers. When the MQB is inserted between an active layer and a cladding layer, electrons having energies higher than a potential barrier formed between the active layer and the cladding layer are reflected by the quantum mechanical effect of the MQB, whereby the overflow of electrons into the cladding layer is suppressed. This result is schematically shown in FIG. 9. In FIG. 9, reference character .DELTA.Ec designates the difference in energy band gaps between the active layer and the cladding layer at the conduction band edge and reference character .DELTA.Ue designates an increase in the effective potential barrier due to the MQB. In this way, electrons are reflected by the potential barrier caused by the MQB even when the electrons have energies higher than .DELTA.Ec and are effectively confined in the active layer.
FIG. 8 is a cross-sectional view showing a semiconductor laser incorporating an MQB, disclosed in the conference digest of 12th IEEE International Semiconductor Laser Conference, 1990, p. 21, PD-10, by K. Kishino, A. Kikuchi, Y. Kaneko, and I. Nomura. In FIG. 8, there are successively epitaxially grown on an n type GaAs substrate 21 an n type GaInP buffer layer 22, an n type AlInP cladding layer 23, a superlattice cladding layer 24, a GaInP active layer 25, a superlattice cladding layer 26, an MQB 27 comprising GaInP wells and AlInP barriers, a p type AlInP cladding layer 28, and a p type GaInP cap layer 29. Preferably, these layers are grown in a gas source MBE process. The superlattice cladding layers 24 and 26 decrease non-radiative recombinations of carriers at the interfaces between the active layer 25 and the cladding layers 24 and 26 to improve the laser characteristics. In this prior art, the MQB was applied to a semiconductor laser for the first time and a reduction in the threshold current and an improvement in the temperature characteristic were achieved by the carrier overflow suppressing effect of the MQB.
In addition, Electronics Letters, 16th Jan. 1992, Vol. 28, No. 2, p.150 discloses a semiconductor laser including an InGaP/InGaAlP multiquantum well active layer and an InGaP/InGaAlP MQB inserted between an InGaAlP guide layer on the active layer and a p type InGaP cladding layer. In this laser structure, a high temperature (90.degree. C.) CW (continuous wave) operation was realized.
In the conventional semiconductor laser devices incorporating the MQBs, the effects of the MQBs are logically and experimentally confirmed. However, the threshold current density of each laser is not sufficiently reduced solely by incorporating the MQB. In addition, the conventional MQBs are not optimized for suppressing an overflow of holes.